Fast melted data.table operations

Question

I am looking for patterns for manipulating data.table objects whose structure resembles that of dataframes created with melt from the reshape

Answer 1

I'm not sure if this is the best way, but you can try:

input[, list(x = sum(value[variable == "x"]), 
             y = sum(value[variable == "y"])), by = "id"]
#    id  x  y
# 1:  1  1  5
# 2:  2  4 11
# 3:  3 15  9